Hi there!
This collision is an example of an inelastic collision since kinetic energy is lost from the collision.
We can represent this using the conservation of momentum formula:
m1v1 + m2v1 = m1vf + m2vf
Where:
m1 = blue ball
m2 = green ball
We know that the final velocity of the blue ball is 0, so:
m1v1 + m2v1 = m2vf
Rearrange to solve for the speed of the green ball:
(m1v1 + m2v1)/m2 = vf
Plug in given values:
((0.15 · 3) + (0.15 · 2)) / 0.15 = 5 m/s